Copper and its alloys have been metallurgical staples for centuries. In particular, brass, some forms of which include an alloy of copper with zinc, is widely used in a number of commercial and consumer applications. Brass is particularly useful for use in plumbing fixtures for potable water systems. Red brass, which comprises zinc, lead, tin, and copper, copper being present in an amount in the range of about 75% to about 95% by weight, may be used in plumbing fixtures. However, yellow brass, because of certain desirable properties, is more commonly used in plumbing fixtures. Yellow brass typically comprises copper and zinc in a ratio of approximately 60% copper to zinc, i.e., a ratio of about 1.5:1.
The essential ingredients in yellow brass are copper and zinc. Other components typically are added to brass alloys to influence their properties. For example, lead has for years been added to brass alloys to enhance, for example, the machinability of the alloys. Machinability may be defined as the ease of cutting of an alloy relative to a standard alloy. "Cutting" may mean turning, shaping, planing, drilling, reaming, tapping, milling, sawing, broaching, or similar operations. See Engineering Metals And Their Alloys, ch. X, p. 472 (1949). In the case of yellow brass, the standard alloy is CDA-360, which has been assigned an arbitrary machinability of 100. Generally speaking, although somewhat dependant on its use, any alloy having machinability of about 75 or more is said to exhibit acceptable machinability.
Another desired property of yellow brass is castability. Castability of an alloy refers to the tendency of the alloy to resist hot tearing, gross shrinkage, intermetallic compound formation and inclusion formation upon casting of the brass in a mold. Castability often is difficult to attain in brass alloys. When an alloy exhibits poor castability, it is commercially impractical to cast the alloy to form plumbing fixtures or other products. The type of casting process also is pertinent to the castability of the alloy; for example, red brass typically cannot be cast in a permanent mold casting process.
Another desired property of a yellow brass alloy is resistance to dezincification. When brass comes into contact with water, the zinc may leach out of the brass, leaving a copper matrix. This copper matrix is brittle, has poor mechanical properties, and suffers in appearance. This can be a particularly serious problem when the alloy is used in decorative products, such as plumbing fixtures.
Polishability is another desired feature of yellow brasses. Preferably, the brass metal, after casting, should exhibit a flawless, nonporous surface, with no hard spot inclusions. After casting, brass typically first is polished with sand paper or an abrasive belt, then is buffed with a buffing compound and wheel to produce a smooth, mirror-like surface on the brass. Porosity or hard spot inclusions detrimentally affect the polishing process, as well as subsequent processes such as plating or clear-coating. Such defects detract from the appearance of a brass decorative product.
Yellow brasses that are easily castable, readily machinable, and resistant to dezincification are known. Most such brasses include lead, an additive which is used to improve both the castability and the machinability of brass. Recently, however, the use of lead in yellow brass alloys has fallen into disfavor. Lead has been identified as posing significant health concerns, and it has been discovered that lead may leach out of the alloy when the brass is used in a plumbing fixture.
Accordingly, brass makers have striven for several years to produce yellow brass alloys that do not contain lead. The first attempts at making a lead-free yellow brass alloy resulted in alloys that were essentially non-castable and poorly machinable. Thus, various other additives have been added in lieu of lead in attempting to enhance the machinability of the brass; for example, bismuth.
Although bismuth is known to enhance the machinability of brass, bismuth is generally regarded as unacceptable in yellow brass alloys. Bismuth is known to greatly increase the brittleness of brass, when used even in very small amounts. See ASM Metals Handbook, pages 907-916 (1948) (discussing the brittleness problem). In addition, bismuth is costly and generally has proven to be less satisfactory than hoped in enhancing the machinability of yellow brass in a castable alloy. For these reasons, the prior art has not been able to effect a satisfactory substitution of bismuth for lead.
Thus, notwithstanding the efforts of those skilled in the art, there exists a need for an effective reduced-lead yellow brass alloy that is readily machinable, castable, and polishable, and that resists dezincification. It is a general object of the present invention to overcome the shortfalls inherent in previously known brass alloys. Specifically, it is a general object of the present invention to provide a reduced-lead yellow brass alloy that exhibits satisfactory castability, machinability, and polishability. It is further a general object of the present invention to provide a non-brittle yellow brass alloy that includes bismuth. It is further an object of the present invention to provide a yellow brass alloy that exhibits minimal dezincification when subjected to water. Further, it is an object of the present invention to provide a yellow brass alloy that has an excellent color and appearance.